Dear researchers

I have a few questions about "bone mineral density calibration" and I would really appreciate it if you could help me with them. I was wondering whether there were any difference between calibrating a HA phantom and k2hpo4 one? when acquisition voltage is changed, the HU values change consequently. do such changes have any effect on equivalent density values? when calibrating we make a linear regression between k2hpo4 concentration and HU values(not the actual density of the solution in the rods) is this correct?